1. Field of the Invention
This invention relates to the molding of sheet material and, in particular, to thermoform molding of plastic sheet material to define overhanging hollow sections therein.
2. Description of the Prior Art
In forming cabinets such as for use in appliances, including refrigerators, freezers and the like, portions of the cabinets, such as portions of the door of the cabinet, may be formed from a plastic sheet which is molded in a thermoforming operation to define a complex, multi-recessed configuration. Certain portions of the configuration may be undercut, presenting a problem in the removal of the thermoformed sheet from the mold, resulting from interference of the mold portion defining the undercut portion of the molded sheet.
On example of such a vacuum thermoformed cabinet wall is illustrated in U.S. Pat. No. 2,784,455 of John W. Pulaski. As shown therein, the door sheet is provided with a recessed portion defined by a movable head pivotally carried on an arm so as to be swung out of the formed recess during the withdrawal of the molded sheet from the mold. A plurality of such movable heads are provided for providing correspondingly a plurality of such recesses, and means are provided for simultaneously retracting each of the heads during the sheet removing operation.
In U.S. Pat. No. 3,126,582 of Lester Scott, a vacuum molding apparatus is shown wherein a rubber disc is provided for defining an annular bead in the thermoformed sheet. The disc is supported centrally so as to flex peripherally during the sheet withdrawing operation, leaving a hollow annular bead in the molded sheet.
Lester Scott, in his subsequent U.S. Pat. No. 3,302,245, discloses an apparatus for forming plastic articles, utilizing a plurality of bar-like segments pivotally connected to each other by a series of links to define a unitarily movable flexible core. The bars are provided with pins extending into slots in end guide plates for guiding movement of the bars during a withdrawal operation wherein the bars move serially along the preselected path defined by the guide plate slots. Relatively expensive apparatus must be provided for effecting the desired withdrawal operation, and the segment, link, pin, and guide structure is similarly complicated and expensive.
Keith W. Kesling, in U.S. Pat. No. 3,467,741, shows a method of making a shelf front such as for use in refrigerator doors, wherein the formed sheet envelopes wooden sticks comprising reinforcing members defining separate folds in the thermoformed sheet. The sticks are retained in the sheet and, thus, form a maintained portion of the formed sheet.